While smooth muscle cell proliferation has been extensively studied, (see, e.g. Schwartz et al., Circulation Research, Vol. 58, No. 4, page 427, the disclosure of which is incorporated by reference herein), the signals controlling the proliferation of smooth muscle cells remain largely unknown. Smooth muscle cell proliferation is known to play a central role in diseases such as arteriosclerosis (atherosclerosis and hypertension). Lack of smooth muscle proliferation in infants also plays a role in vascular malformations. This failure of smooth muscle cell replication results in untreatable vascular lesions which often lead to death.
Although it is now generally acknowledged that proliferation of smooth muscle cells occurs during formation of atherosclerotic lesions, the role of that proliferation response in the overall history of the plaque is not all obvious. A few investigators have suggested that replication occurring during development of arteries is the initial event in formation of atherosclerotic lesions, preceding lipid accumulation or endothelial injury.
The major hypothesis explaining smooth muscle replication in the vessel wall is the response-to-injury hypothesis. In brief, the hypothesis is that smooth muscle cells in the wall normally exist in a quiescent state. When the endothelium is injured, platelets release a factor or factors that stimulate smooth muscle cell movement into and replication within the arterial intima (Ross, Arteriosclerosis 1:293-311, 1981). Ross also showed that the cultured smooth muscle cells require a platelet derived growth factor (PDGF) for proliferation (Ross and Glomset, N. Eng. J. Med. 295; 369-377 and 420-425, 1976). The apparent conclusion is that platelet release is necessary for smooth muscle proliferative response to balloon denudation.
Ross's observation led to the ensuing purification of the PDGF, identification of its receptor and, more recently, identification of the oncogene c-sis as the gene for one of the two PDGF peptide chains.
The second known requirement for cell cycle progression is availability of somatomedin C., also known as insulin-like growth factor (IGF-1). IGF-1 itself can be synthesized by smooth muscle cells, and antibodies to IGF-1 inhibit cell cycle progression. These data suggest that PDGF is capable of stimulating production of its own progression factor. This observation is of considerable importance to the interesting possibility that smooth muscle replication may be controlled by factors intrinsic to the vessel wall.
Other substances mitogenic for smooth muscle cells, apart from PDGF have also been studied. In addition, platelets also contain a protein resembling epidermal growth factor (EGF) (Oka and Orth, J. Clin. Invest. 72:249-259, 1983) and Assoian et al., 1984) and a factor able to assist growth of cell in suspension called B tumor growth factor (Tucker et al., Science 226: 705-777, 1984). The relative contribution of each of these to stimulation of proliferation is largely unknown.
The stimuli controlling smooth muscle replication in hypertension also remains largely unknown. PDGF may play an important role in microvascular changes in malignant hypertension, but is not likely to be involved in large vessels or in any vessel affected by milder and more chronic forms of high blood pressure.
While there has been much research on the role of smooth muscle in various disease pathologies, and several mechanisms and roles of growth factors such as PDGF have been explored, there continues to be a need for new information about mitogens which stimulate the proliferation of smooth muscle cells. The identification of such mitogens will permit various treatment strategies to be devised such as competitive binding strategies employing antibodies to the smooth muscle mitogen or competitive proteins which will bind to the receptors for such mitogens. Smooth muscle mitogens may also be used in the treatment of conditions such as vascular malformation or as a growth factor in wound/ulcer healing.